1. Field of the Invention
The present invention relates to a power tool employing a brushless motor.
2. Description of the Related Art
Brushless motors have commonly been employed in power tools to achieve a more compact design because brushless motors do not require an electrical connection using a brush and commutator for a rotor mounted on a rotational shaft. However, the relatively large power loss that occurs when the brushless motor is driven produces heat that can affect the motor operations, leading to a reduction in efficiency or malfunctions. In fact, loss in a stator unit is a primary source of heat generation and comprises copper loss occurring when an electric current flows through the stator coil, and iron loss occurring in the stator core material due to changes in magnetic flux density. Consequently, cooling mechanisms have been proposed for a variety of stators in conventional brushless motors. A cooling mechanism for a conventional stator disclosed in Japanese unexamined patent application publication No. 2004-274800, for example, comprises first openings formed in a motor housing for drawing external air into the housing, and second openings formed in the housing on the opposite side of the stator from the first openings in the axial direction of the rotational shaft for exhausting cooling air from the housing. A fan integrally mounted on the rotor draws air into the housing and generates, in particular, a flow of air through the stator coil, thereby directly cooling the stator coil area.
Further, output transistors in the brushless motor constituting switch elements for an inverter circuit board (motor-driving circuit board) generates a large amount of heat when supplying a large current drive signal to the stator coil. Hence, measures must be taken to cool these components. Japanese unexamined patent application publication No. 2005-102370 proposes a mechanism for cooling the inverter circuit board, along with the stator unit of the motor, by disposing these components in a channel along which cooling air flows through the motor housing.
However, while the above mechanisms are capable of cooling the stator and inverter circuit board of a brushless motor by introducing external air, circulating external air through the stator invites dust particulate in the air to accumulate on the rotor and stator.